1. Technical Field
The present disclosure relates to a system and method for measuring semiconductor patterns, and more particularly to a system and method for measuring a dimension of patterns formed on a photomask.
2. Discussion of Related Art
A fabricating process of semiconductor integrated circuits includes photolithography for transferring a circuit pattern formed on a photomask onto a photoresist layer coated on a wafer. A photoresist pattern formed using photolithography is used as a mask for etching an underlying material layer. A critical dimension (CD) of the photoresist pattern on the wafer is a factor to determine an integration density of semiconductor devices. The integration density affects the prices of the semiconductor devices. Thus, reduction of a CD of the wafer photoresist pattern has been studied.
Since a linewidth uniformity of the photoresist pattern on the wafer affects a yield of semiconductor devices, various technologies have been suggested for improving the linewidth uniformity as the demand for smaller and denser integrated circuits (ICs) increases. Since the photoresist pattern on the wafer is the result of transfer using photolithography, shapes and characteristics of the photoresist pattern on the wafer are affected by shapes and characteristics of a corresponding photomask. Due to a non-linear characteristic during the transfer, a linewidth of the photoresist pattern on the wafer varies at a higher reduction ratio than a reduction ratio of transfer. Thus, a photomask used in photolithography needs to pass a test including factors such as linewidth uniformity.
Conventionally, an image of a pattern or a reflective spectrum reflected from the pattern is used in measuring a linewidth uniformity of the photomask. A method using the image is illustrated in FIG. 1. Referring to FIG. 1, after an image 10 is displayed on an optical device such as a monitor, a pattern dimension is measured using the phenomenon that a brightness (or luminance) of the image varies at a boundary of the pattern. Since a measurement error is determined by a photographing resolution, a photographing method using a resolution to distinguish pattern dimensions smaller than a required measurement error is needed. To achieve a high resolution, the method using the image 10 employs a scanning electron microscope (SEM).
Although circuit patterns are designed with the same shape, their dimensions may vary based on their locations. Accordingly, a large number of measurements are needed to obtain an accurate measuring result. The measuring method using SEMs has a non-negligible measurement error and requires long measuring time. Thus, increased time and cost are needed for obtaining an accurate measuring result.
Another conventional method using a reflective spectrum is illustrated in FIG. 2. Referring to FIG. 2, the method includes using reflective spectrum-pattern dimension data prepared from design data of the photomask to indicate a relationship between a reflective spectrum and a pattern dimension. Since the reflective spectrum is the result of a complex reflection procedure, valid reflective spectrum-dimension data based on a theoretical access may be obtained only in limited cases (e.g., regularly repeated linear patterns shown in FIG. 3). Therefore, an empirical method is needed to obtain valid reflective spectrum-dimension data for patterns of complex shapes, as shown in FIG. 4 or FIG. 5.
The empirical method includes, for example, determining the reflective spectrum-dimension data using a different test mask. However, the empirical method is incapable of generating valid reflective spectrum-dimension data if pattern shapes are too complex. As a result, methods using the reflective spectrum are used in a photomask having circuit patterns which are as simple as shown in FIG. 3 (e.g., ISO mask for defining an active region).